I started just signing what I'm talking about.
We started with milk, dog, cat, more, finished, bath. We're still waiting for all of these to come together but I keep signing new things to DS all the time.
BTW, my DS is 9 1/2 months and they say to start at about 8, but I started sooner than that.

I started to sign with my dd when she was 4 months old, just using 6 signs right now. I got a book from the library called Signing Smart with Babies and Toddlers: A Parent's Strategy and Activity Guide - it is a really quick and fast read and lays it all out in an easy to follow way - here's more info on them: http://www.wideeyedlearning.com/

I started with both of my children when they were about four months old. We started with milk, more, and all done, primarily. Basically, I just sign every word I know the sign for as well as speak it. DS is just 8 months old and will sign milk to me some of the time.